[FFmpeg-devel] [PATCH] add signature filter for MPEG7 video signature

[Gerion Entrup]

On Sonntag, 20. März 2016 17:01:17 CET Thilo Borgmann wrote:
> > On Sun, Mar 20, 2016 at 12:00:13PM +0100, Gerion Entrup wrote:
> [...]
> 
> > This filter does not implement all features of MPEG7. Missing features:
> > 
> > - binary output
> > - compression of signature files
> 
> I assume these features are optional?
Compression is optional (could be set as flag in the binary representation). I
have not found, whether binary output is optional. It is definitely possible to
only work with the XML-Files.

> 
> > - work only on (cropped) parts of the video
> 
> How useful is this then? Has fingerprint computed only on (cropped) parts of
> the video any value outside of FFmpeg itself - does this comply to the spec
> so that it can be compared with any other software generating it?
To clarify, the filter does not crop anything. The standard defines an optional 
cropping to, I guess, concentrate on specific video parts (this is not 
implemented). Assuming someone is recording a monitor, then e.g. the unrelated 
part of the video could be cropped out. Beside that, the signature itself is 
invariant to cropping up to a certain limit.

The cropping values (upper left and bottom right position are specified in the 
xml, so another software could either crop the same way or compare only with 
the cropped input.
(The fact, that ffmpeg has a cropping filter, would make such a feature some 
kind of redundant.)

The XML is standard compliant. The signature is not bitexact. 3-4 (ternary) 
values in the framesignature differ from the signature of the sample files, but 
the conformence tests [1] allow up to 15 ternaryerrors. I'm not exactly sure 
why the difference occur, I guess, this are floating point issues, because my 
implementation calculates the values with a slightly different method than the 
reference software. Currently, I don't have the sample files anymore. If there 
is interest, I will try to get them again.

[1] https://www.itscj.ipsj.or.jp/sc29/open/29view/29n11696t.doc